Vaccination against disease is effected by inducing a protective immune response to a pathogenic organism without causing disease. One of the most efficient means of accomplishing this for pathogenic viruses is to modify the genome of the virus so that it can grow in a human or other animal with reduced disease symptoms, but nonetheless induce an immune response, which will protect the individual from infection by the actual pathogen. Examples of such live attenuated virus vaccines include those for polio, measles, mumps, rubella, chicken pox and smallpox. In the past, however, such vaccines have been derived empirically from the pathogenic virus. It would be desirable to have a method of engineering safer life viruses directly.